How high of a percentage of non-wool-ish fiber can you use and still have it felt? I saw a yarn that was 74% alpaca, 22% wool and 4% nylon, and another that was 70% alpaca and 30% silk. Will either of those felt?

Or, if you don't want to felt your project, and don't want to worry about it accidently getting thrown in the wash by a well meaning dh or mom (ask me how I know...sigh....) how low of a non-felting fiber mixed with wool can you use?

Is that all clear as mud?

Alpaca felts, so both of the yarns mentioned could felt very well. The nylon or silk would kind of crumple up, since it wouldn't felt. My fiber arts guild did some felted baby booties for a member's new baby out of a wool-silk blend and they felted just fine.
As for the second question, some wools felt more than others. There are sheep breeds that have kind of puffy, spongy wool that doesn't felt so much. There is also superwash wool that is chemically treated to prevent felting.